Increases in transient receptor potential vanilloid‐1 mRNA and protein in primary afferent neurons stimulated by protein kinase C and their possible role in neurogenic inflammation

Abstract

A recent study by our group demonstrates pharmacologically that the transient receptor potential vanilloid-1 (TRPV1) is activated by intradermal injection of capsaicin to initiate neurogenic inflammation by the release of neuropeptides in the periphery. In this study, expression of TRPV1, phosphorylated protein kinase C (p-PKC) and calcitonin gene-related peptide (CGRP) in dorsal root ganglion (DRG) neurons were visualized using immunofluorescence, real-time PCR and Western blots to examine whether increases in TRPV1 mRNA and protein levels evoked by capsaicin injection are subject to modulation by the activation of PKC and to analyze the role of this process in the pathogenesis of neurogenic inflammation. Capsaicin injection into the hindpaw skin of anesthetized rats evoked increases in the expression of TRPV1, CGRP and p-PKC in mRNA and/or protein levels and in the number of single labeled TRPV1, p-PKC and CGRP neurons in ipsilateral L4–5 DRGs. Co-expressions of TRPV1 with p-PKC and/or CGRP in DRG neurons were also significantly increased after CAP injection. These evoked expressions both at molecular and cellular levels were significantly inhibited after TRPV1 receptors were blocked by 5′-iodoresiniferatoxin (5 μg) or PKC was inhibited by chelerythrine chloride (5 μg). Taken together, these results provide evidence that up-regulation of TRPV1 mRNA and protein levels under inflammatory conditions evoked by capsaicin injection is subject to modulation by the PKC cascade in which increased CGRP level in DRG neurons may be related to the initiation of neurogenic inflammation. Thus, up-regulation of TRPV1 receptors in DRG neurons seems critical for initiating acute neurogenic inflammation.

Protease-Activated Receptor 4: A Critical Participator in Inflammatory Response

“…PAR4 protein and mRNA are commonly observed in small nociceptive dorsal root ganglion (DRG) neurons, the majority of which are peptidergic and are marked by substance P and calcitonin gene-related peptide (CGRP) [73,89]. Furthermore, increasing experimental results prove that activated nociception and inflammation induce a rapid increase in protein and mRNA expression of CGRP in DRG neurons [90][91][92]. The expression of CGRP in DRG neurons plays a vital role in regulating neurogenic inflammation and pain [72,93].…”

New insights into protease-activated receptor 4 signaling pathways in the pathogenesis of inflammation and neuropathic pain: a literature review

“…25 Furthermore, it is expressed in neurons, particularly in the sensory neurons isolated from the dorsal root ganglion (DRG) that express the sensory neuropeptides CGRP and SP. Accumulating evidence demonstrates that nociceptor activation and inflammation evoke a rapid change in the levels of CGRP mRNA and protein in DRG neurons, [26][27][28] and CGRP expression in these neurons contributes to the regulation of neurogenic inflammation and pain. [29][30][31][32] In a previous study, the PAR4 agonist did not induce a calcium signal in DRG neurons; it reduced the calcium signal of DRG neurons in response to KCl, 16 suggesting that PAR4 activation could inhibit the nociceptive signal in DRG neurons.…”

“…31 Capsaicin-activated TRPV1 is involved in noxious responses and induction of TRPV1 and CGRP mRNA and protein expression in DRG neurons. 28 Thrombin or PAR4-AP causes activation of the PLCb/PKC pathway: intracellular calcium release, sensitization of TRPV1, and translocation of the epsilon isoform of PKC (PKCe) to the neuronal cell membrane. 33 Apart from PKC, PKA is essential for mediating phenyl glycidyl ether 2 (PGE2)-induced modulation of TRPV1.…”

“…31 Several studies have shown that these rapid changes in CGRP mRNA and protein levels were evoked by nociceptor activation and inflammation. 26,28 PAR4-AP induces the upregulation of CGRP expression in primary sensory neurons via activation of the ERK1/2 signaling pathway. Notably, this upregulation was significantly inhibited after inhibition of ERK1/2 phosphorylation, 32 intraplantar injection of PAR4-AP, or treatment of cultured DRG neurons with PAR4-AP evoked a significant increase in the number of DRG cells expressing CGRP and cytoplasmic and nuclear staining for phospho-ERK1/2 (p-ERK1/2).…”

“…46 The mechanism by which capsaicin activates TRPV1 receptors causes primary afferent neurons in the DRG to enhance the expression of TRPV1 and CGRP at the mRNA and protein levels. 28 …”